As used herein, the term ‘dynamic range’ (DR) may relate to a capability of the human psycho-visual system (HVS) to perceive a range of intensity (e.g. luminance, luma) in an image, e.g. from the darkest darks to the brightest brights. In this sense, DR relates to a ‘scene-referred’ intensity. DR may also relate to the ability of a display device to adequately or approximately render an intensity range of a particular breadth. In this sense, DR relates to a ‘display-referred’ intensity. Unless a particular sense is explicitly specified to have particular significance at any point in the description herein, it should be inferred that the term may be used in either sense, e.g. interchangeably.
As used herein, the term high dynamic range (HDR) relates to a DR breadth that spans the some 14-15 orders of magnitude of the human visual system (HVS). For example, well adapted humans with essentially normal vision (e.g. in one or more of a statistical, biometric or ophthalmological sense) have an intensity range that spans about 15 orders of magnitude. Adapted humans may perceive dim light sources of as few as a mere handful of photons. Yet, these same humans may perceive the near painfully brilliant intensity of the noonday sun in desert, sea or snow (or even glance into the sun, however briefly to prevent damage). This span though is available to ‘adapted’ humans, e.g. to those whose HVS has a time period in which to reset and adjust.
In contrast, the DR over which a human may simultaneously perceive an extensive breadth in intensity range may be somewhat truncated, in relation to HDR. As used herein, the terms ‘visual dynamic range’ or ‘variable dynamic range’ (VDR) may individually or interchangeably relate to the DR that is simultaneously perceivable by a HVS. As used herein, VDR may relate to a DR that spans 5-6 orders of magnitude. Thus while perhaps somewhat narrower in relation to true scene referred HDR, VDR nonetheless represents a wide DR breadth. As used herein, the term ‘simultaneous dynamic range’ may relate to VDR.
Until fairly recently, displays have had a significantly narrower DR than HDR or VDR. Television (TV) and computer monitor apparatuses that use typical cathode ray tube (CRT), liquid crystal display (LCD) with constant fluorescent white back lighting, or plasma screen technology may be constrained in their DR rendering capability to approximately three orders of magnitude. Such conventional displays thus typify a low dynamic range (LDR), also referred to as a standard dynamic range (SDR), in relation to VDR and HDR.
Advances in their underlying technology however allow more modern display designs to render image and video content with significant improvements in various quality characteristics over the same content as rendered on less modern displays. For example, more modern display devices may be capable of rendering high definition (HD) content and/or content that may be scaled according to various display capabilities such as an image scaler. Moreover, some more modern displays are capable of rendering content with a DR that is higher than the SDR of conventional displays.
Such “HDR displays” as they are often called (although actually, their capabilities may more closely approximate the range of VDR) and the DR extension of which they are capable in relation to conventional SDR displays, represent a significant advance in the ability to display images, video content and other visual information. The color gamut that such an HDR display may render may also significantly exceed the color gamut of more conventional displays, even to the point of capably rendering a wide color gamut (WCG). Scene related HDR or VDR and WCG image content, such as may be generated by “next generation” movie and TV cameras, may now be more faithfully and effectively displayed with the “HDR” displays (hereinafter referred to as ‘HDR displays’).
As with the scalable video coding and HDTV technologies, extending image DR typically involves a bifurcate approach. For example, scene referred HDR content that is captured with a modern HDR capable camera may be used to generate an SDR version of the content, which may be displayed on conventional SDR displays. In one approach, generating the SDR version from the captured HDR version may involve applying a tone mapping operator (TMO) to intensity (e.g. luminance, luma) related pixel values in the HDR content. In a second approach, as described in International Patent Application No. PCT/US2011/048861 filed 23 Aug. 2011, herein incorporated by reference for all purposes, generating an SDR image may involve applying an invertible operator (or predictor) on the HDR data. To conserve bandwidth or for other considerations, transmission of the actual captured HDR content may not be a best approach.
Thus, an inverse tone mapping operator (iTMO), inverted in relation to the original TMO, or an inverse operator in relation to the original predictor, may be applied to the SDR content version that was generated, which allows a version of the HDR content to be predicted. The predicted HDR content version may be compared to originally captured HDR content. For example, subtracting the predicted HDR version from the original HDR version may generate a residual image.
The generated SDR content may be encoded by a base layer (BL) encoder. Similarly, the residual image may be encoded by an enhancement layer (EL) encoder. To improve compatibility with legacy encoders, a quantizing function may be applied to the residual image prior to encoding to lower the bit depth of the residual image to be compatible with the enhancement layer (EL) encoder.
The encoded SDR content may be transmitted as a base layer signal and the encoded quantized residual image as an enhancement layer signal. Moreover, metadata may be transmitted that comprises parameters required for the prediction process. The combined data may be transmitted in a single bitstream. This approach typically consumes less bandwidth than would be consumed in sending both the HDR and SDR content directly into the bitstream. Compatible decoders that receive the bitstream sent by the encoder may decode and render the SDR on conventional displays. Compatible decoders however may also use the residual image, the SDR image and the metadata to reconstruct a HDR version of the SDR content for use on more capable displays.
In such layered HDR coding, the residual bitstream may require more than the traditional 8-bits per color pixel for adequate representation. Without any preprocessing, direct coding of a HDR residual using a traditional SDR compressor, such as those described by the MPEG coding standards, may cause severe picture artifacts, such as blockiness and banding.
In an approach, as described in International Patent Application No. PCT/US2012/034747 filed 24 Apr. 2012, herein incorporated by reference for all purposes, the residual image is pre-processed by a non-linear quantizer before being encoded. Such companding (or compansion) of the residual HDR signal allows a subsequent encoder to operate more efficiently and reduces coding artifacts.
Despite the improvements achieved by using a non-linear quantizer, a continuing demand exists to optimally reconstruct the HDR image at the decoder without consuming excessive bandwidth.
The approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated, it should not be assumed that any of the approaches described in this section qualify as prior art merely by virtue of their inclusion in this section. Similarly, issues identified with respect to one or more approaches should not assume to have been recognized in any prior art on the basis of this section, unless otherwise indicated.